The conventional dumbbell of the most primitive type is made integrally of a metal material by casting and is therefore provided with a fixed weight. The conventional dumbbell of the more advanced type is provided with a plurality of round weights joined by a short bar by which it is lifted or swung about in the hand for muscular exercise. Such a dumbbell is therefore adjustable in weight. The round weights of the dumbbell are movable and can therefore accidentally fall on the ground to inflict a wound on the dumbbell user or bystanders.
With a view to overcoming the drawbacks of the conventional dumbbells described above, a prior art dumbbell is provided with a plurality of rectangular weights. The number of weights that can be lifted along with the dumbbell main body can be manipulated by changing the position of a fastening pin in a groove. In light of the fastening pin being located insecurely in the groove, the weights can accidentally fall on the ground at the time when the dumbbell is in use. In addition, after a prolonged use of the dumbbell, the fastening pin is vulnerable to deformation, thereby causing the fastening pin to be unable to press securely against the connection rod of the weights.